Ok, here's the game (challenge?). See how long it takes you to 'find'
the 3D image from the picture pair on the dddphoto home page!

First, here is some instruction using simple line drawings that portray
the process. You will not see 3D in these drawings, but they will
help orient you to the method.

Position yourself comfortably in front of the screen, looking straight
toward the center. The following drawing depicts what you see while
focused on the pair of rectangles

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Now, relax your eyes, "stare past the screen" as if at a virtual object far away.
Notice how the two images "split" within your field of view.
When your eyes are both looking straight forward, focused distantly, you should
observe only three images within your field of vision, as indicated in the
next figure.

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The middle one would appear to be 3D if you actually do this with a real
photo pair.

The animation below simulates what crosses your field of vision during
this process.

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This is a bit confusing, but practicing a few times will be worth the discovery.